Since patch 4.1 went live, I have been noticing some irregularities with my mana bar. I don't check it too often, as I'm usually focused on my raid's health instead. As expected, I watch my mana the most when it's running low. It seemed as if I was both gaining and spending more mana than usual, and so I started doing some research. A few threads on the WoW official forums revealed that there were others seeing the same sort of issues and that Seal of Insight and Judgement were the main suspects.

The initial reports that I found indicated that holy paladins were actually being robbed of mana after using Judgement. We normally gain mana after Judging, due to its interaction with Seal of Insight. The issue was that when casting a spell after Judging, we'd somehow lose the mana that we had just gained. Judgement actually costs us some mana, though typically we recoup the cost from the mana it grants. If we're not seeing a return from Judgement with SoI active, then we should actually stop Judging on cooldown. Luckily for us, that's not the whole story of what's happening.

You're not losing any mana

While there are a lot of sensational posts claiming that the sky is falling for paladins; that's simply not the case. I started doing my own testing of the SoI/Judgement mechanic to confirm what I was reading. While it's good to listen to members of the class community to gain a different perspective, we also have to remember to confirm things independently. In order to get the full story, I tested using a variety of methods, and I'm pretty sure that I have figured out exactly what's occurring. One note is that this bug affects any paladin using Seal of Insight, though that's typically only holy paladins.

Incorrectly reporting the mana gain

The first issue is that when we use Judgement with Seal of Insight active, our mana bar jumps up by around 7,000 mana. If you've been playing a holy paladin during Cataclysm, you know that we usually get about 3,500 mana back from Judgement. Our combat log shows us getting about 3,500 mana, but the mana bar itself shows a 7,000 mana boost. I've seen this issue on more than one paladin and with several different unit frames addons. WoW is simply incorrectly reporting the amount of mana we gained via the mana bar.

A bug like this can be hard to catch, as we don't actually get any extra mana, and it won't show up on World of Logs since the combat log reports the correct value. In order to test this thoroughly, I defined a plan to reproduce the effect. Basically, burn through 10,000 or so mana, take note of your current mana value (via the mana bar), and then use Judgement. After Judging, you should immediately have about 6,000 more mana than when you Judged. The reason we see a 6,000 mana increase instead of 7,000 is that Judgement actually costs some mana to use. You can also run yourself completely out of mana and then spam Judgement as soon as you have enough mana to, and you'll see yourself end up with 7,000 mana immediately.

To show this graphically:

You start with 0 mana in your pool by healing until you run out.

You use Judgement as soon as you can, which consumes all of your recently generated mana.

You appear to have 7,000 mana on your bar, but you're unable to cast a spell that costs 7,000 mana. That's because you only have 3,500 mana.

Once you really have 7,000 mana (which will be displayed as 7,000 + 3,500, or 10,500), you can cast a spell that costs 7,000 mana.

Because the mana bar is incorrectly reporting the mana gain, your action bar can get confused. The action bar checks the mana bar to see if you have enough mana to use a spell and lights up those buttons that you have enough mana to use. If you run yourself completely out of mana and use Judgement, your mana bar shows around 7,000 mana. Divine Light costs right around 7,000 mana, so the action bar lights up that button. When you try to use that button, you get the familiar "Not enough mana" error. In fact, since WoW knows behind the scenes that you actually only have 3,500 mana, you have to wait until you actually have 7,000 mana to cast Divine Light or any other spell.

Correcting the anomaly

WoW detects the disturbance in the force and makes plans to correct the problem. We clearly don't actually receive 7,000 mana from Judging, as we tested by trying to cast a spell with that "bonus" mana. In order to fix the mana bar, WoW needs to knock 3,500 mana out of our displayed mana pool. Instead of just removing that excess mana on the fly, WoW waits for us to cast our next spell. As soon as we cast a spell -- any spell -- the game simply removes bonus mana from what's displayed on our bar. Your spell didn't actually cost any extra mana, but that's what your mana bar would lead you to believe.

While I can understand the thought process behind the design, it also makes us think that we're losing mana when we're not. You could cast a Divine Light for about 7,000 mana and then see 10,000 mana deducted from your bar; the anomalous mana is tacked onto the initial cost of a heal. The interaction between casting and our mana bar dropping becomes very important when we're low on mana, which makes our mana bar incredibly unreliable. It could report too much mana if we just Judged, and we could find ourselves unexpectedly out of mana after casting. While it's important that the mana display bug is corrected quickly, having it happen on our next cast makes us very suspicious of what's actually occurring.

Testing mechanics thoroughly

If you suspect that some mechanic isn't working as intended, especially after a major patch, the best thing you can do is to test. Thorough testing will let you figure out the truth behind what's occurring. Having numbers to back up your claims is also key to getting your voice heard in the community, as most won't simply accept any statements at face value.